In relation to providing desired speed control for a large direct current drive motor, such as the drive motor for a reversing slab mill, it is sometimes desired to accelerate the drive motor from zero speed to rated run speed in a time period of about 3 seconds. The applied speed reference signal to the motor speed control apparatus is increased accordingly, and it is compared with an actual speed feedback signal to establish a speed error condition signal which is amplified and applied to the voltage supply operative with the drive motor armature. This modifies the motor armature voltage as necessary to correct any speed error condition.
In the operation of the prior art speed control apparatus, the typical speed reference signal rapidly comes up a ramp and the motor speed goes up accordingly. At the dynamic crossover at base speed, the field flux begins to weaken but not fast enough to avoid motor armature voltage overshoot. If for purpose of illustration, it is assumed that the typical drive motor, of about 10,000 to 15,000 horsepower rating to be controlled in its speed of operation, is designed to be operated for about 20 years of normal life, and if undesired over terminal voltage operation is permitted in relation to rapid speed changes, this could result in motor deterioration such that only 10 years or so of satisfactory operational life might be obtained and additional maintenance and upkeep cost could be realized.
A prior art speed regulated electrical drive is shown in a published article which appeared in the Iron and Steel Engineer yearbook for 1966 at pages 659 to 667. The motor control system consisted of two regulators including a speed regulator operating on the armature power supply and a counter EMF regulator operating on the motor field. The counter EMF regulator consisted of a motor field flux regulator and a counter EMF preamp. At values below rated counter EMF of the direct current motor, the regulator is biased to hold the motor at full field flux. Above base speed of the motor the counter EMF of the drive is clamped at its rated value and the motor field excitation is decreased to hold this value as the motor speed is increased. This results in the motor being operated at maximum field and therefore maximum available torque and power for any operating speed. The speed controller consists of two loops, an inner loop armature current controller and an outer speed control loop, with the armature current limit obtained by limiting the speed controller output which represents a reference of the armature current controller. The speed controller has a proportional plus integral response characteristic, and provides together with the integral relationship between speed and current a double integrating speed control loop, which responds to changes of the speed reference signal as a zero velocity error system.
In U.S. Pat. No. 3,497,779 of H. Eisele, assigned to the same assignee, there is disclosed a parallel controller arrangement, including a voltage controller and a current controller, operative to control the armature voltage of a direct current motor. The voltage controller responds to a desired speed reference signal for controlling the motor armature current. When the armature current exceeds a predetermined current limit value, the current controller assumes control over the voltage controller and effectively provides a maximum value of controlled armature current.